1. Field of the Invention
This invention relates generally to an apparatus for sensing and measuring the flow of dry particulate material, and more particularly to such apparatus including a probe defining an electrode which is adapted for placement within a fluid conduit for sensing the flow of dry particulate material along such fluid conduit. Such particulate material may be tiny in size and not visible to the eye under ordinary lighting conditions.
2. Description of the Prior Art
When dry particulate material or particles impact or rub against an object such as a metal probe, the friction of the particles passing over the probe induces a measurable electrical charge transfer to the surface of the probe. Such charge transfer is known as triboelectric effect. The amount of charge imparted to the probe is related to the flow rate of such particles. By measuring such electrical charge, the existence of the flow and the flow rate of the particles may be determined or sensed. Upon the detection of an excessive flow rate of particles past the probe, a signal such as an audible alarm may be provided, or the signal may be used to control a valve, etc.
To provide accurate measurements of flow, sources of noise charge induced on the probe must be minimized. Stray electrical interference or electromagnetic radiation directed toward the probe can render the measurements of the probe inaccurate. Electrical switches and other equipment in the area such as fans and blowers used in conduits and elsewhere to control the flow of the dry particulate material can emit electromagnetic radiation which can generate noise charges on the probe.
At least two prior art dry material flow switches are commercially available and are part of the prior art related to the invention. The first is a product designated as FS10000 Dry Material Flow Switch sold by BABBITT International, Inc., of P. O. Box 70094, Houston, Tex. 77270. The FS 10000 switch includes a housing and bushing with an electrically insulated rod of stainless steel extending therefrom. The bushing enables the housing to be secured (e.g., by threads) within an opening of a conduit such that the rod extends perpendicularly into the flow path of the conduit. A coaxial cable is conductively attached to the end of the rod within the housing. The coaxial cable, electrically insulated from the housing, runs to a remote charge sensing electrical circuit for producing an alarm or other signal depending on the level of charge sensed on the cable.
The second is a switch designated as Model No. 2100 of Auburn International, Inc. of Eight Electronics Avenue, P. O. Box 2008, Danvers, Mass. Such switch likewise includes a probe extending from a housing, with a bushing provided to screw the housing to an opening in a conduit so that a probe, electrically insulated from the housing, may be inserted perpendicularly into the flow path of the conduit. The probe of the Auburn Model No. 2100 is likewise converted to an electronic sensing device which senses the level of charge transferred to the probe because of the triboelectric effect of dust or small particulate matter flowing against and past the probe in the conduit.
U.S. Pat. No. 4,619,145 dated Oct. 28, 1986 describes apparatus for sensing and measuring the flow of dry particulate material. It recognizes the problem of electrical interference on electrodes or probes for a small diameter critical flow path or passage for dry particulate material. Insulators are provided in the '145 patent for insulating an electrode from electrical interference when such electrode is in contact with the dry particulate material.